Microwave cooking is based on the ability of microwaves to interact with the components of a food product and generate heat energy. The amount of interaction and subsequent heat that is generated is related to the composition of the food and the specific heat of each ingredient. Food molecules which carry a dipoler electrical charge will vibrate as they align themselves with the rapidly fluctuating electric field. This causes heat of friction within the molecules. Since water carries a di-poler charge, foods containing high contents of water will generate a great deal of heat due to the reactiveness of water in a microwave.
Although microwave cooking is greatly influenced by the dielectric properties of ingredients, it is also affected by the food product's physical state, density, size, shape and thickness.
The greater a food product's density, the greater the microwave energy absorption and the lower the microwave penetration depth. Also, the shape of food and uneven patterns in a microwave can result in nonuniform temperature distributions.
In intermediate to low moisture foods such as baked goods, dramatically different patterns of starch transformation can exist which normally are responsible for poor textures such as toughening and cracking. This is known as staling. The primary problem is that dehydration cannot occur at the surface since the water within the product is continually being converted to steam and migrating out, causing evaporative cooling and condensation at the surface. This provides a wet or soggy surface texture. Water boils at 100.degree. C. and its heat exchange capability diminishes as it evaporates as steam. The ability of ingredients to evenly distribute heat in a microwaveable bread will depend on both their specific heat and their relationship with water.
When starch imbibes water during gelatinization, increased heating rates will occur in that location due to interaction of microwave radiation with water. Because more intense heat results from microwave energy, starch's tolerance to textural breakdown is narrowed.
A critical moisture level exists for most starches below which gelatinization will not take place. This is usually around 30%. Gelatinized or swollen starch binds less water to its structure; therefore, more water is free to respond to the alternating microwave field resulting in heat build up and undesirable moisture migration.
Microwave heating of bread products results in the problem of rapid staling or toughening. This is most likely due to the non-uniform heating and increased range of starch gelatinization which takes place at the moisture level of the bread. Toughening or harsh chewiness of a microwaved bread product is attributed to accelerated staling, which is related to starch recrystallization and moisture migration.
Tests were conducted on commercially available precooked hamburger buns to determine the extent of moisture migration. The results were as follows:
TABLE I ______________________________________ Commercial Hamburger Buns Location % Before % After of Moisture Microwaving Change ______________________________________ Top Third 29.02 35.15 +6.13 Middle Third 34.67 34.86 +0.19 Bottom Third 36.31 29.99 -6.32 Total Moisture 100.00 100.00 Total moisture loss was 13.12% ______________________________________
A noticeable staling and toughening of the product was noted.